In the course of practicing a wide variety of commercially important industrial processes, aqueous process streams (or more generally “aqueous systems”) are produced which are contaminated with pernicious quantities of mercury present as colloids, ions or organically bound compounds, and with hydrocarbons which are solubilized, dispersed, or emulsified in the aqueous system. The mercury contaminants have proved to be particularly difficult to remove or reduce to levels deemed harmless to humans. This is a problem that has reached very serious dimensions, in part because even where mercury may initially be present in low concentrations in sea water, the natural predator-prey relation in the oceanic food chain causes build up of mercury in the large fish species widely eaten by humans to a level where e.g. certain types of tuna are now considered unsafe for frequent human consumption, especially by women who are pregnant.
Generally similar concerns exist with respect to hydrocarbon contaminants. Many of these latter noxious contaminants are also among the more difficult compounds to remove from aqueous media. Most are carcinogenic, and yet such compounds must be removed or minimized before discharge of the industrial stream in which they may be present in order to environmentally protect the bodies of water which the discharges eventually reach.
One type of such hydrocarbon contaminant consists of dispersed oils, which are often present as oil-in-water emulsions. A further source of contamination arises from presence in the water of pernicious slightly soluble organic compounds such as benzene, toluene, xylene, halogenated hydrocarbons, ethoxylated glycols, etc.
In the present inventor's U.S. Pat. No. 6,180,010 it is disclosed that the compositions described in the inventor's U.S. Pat. Nos. 5,437,793; 5,698,139; 5,837,146; and 5,961,823, (all of which disclosures are hereby incorporated by reference) have extremely strong affinities for the aforementioned contaminants in water; and that when aqueous streams containing these noxious contaminants are passed through filtration media incorporating these compositions, the contaminants are immobilized at the media, as a result of which concentration levels of the contaminants in the filtrate may be reduced to very low values. The principles of these earlier inventions are also applied in the present invention, but in a synergistic combination with further contaminant removal methodology.
An important example of the type of operation to which the present invention is directed arises in the operation of oil and gas exploration and drilling facilities maintained upon platforms in natural or other bodies of water, including so-called “off shore drilling platforms”. In conducting operations at these platforms industrial streams are developed (often referred to as “produced water”) which by virtue of the processes conducted contain comparatively large quantities of mercury and various organic contaminants, including those mentioned above.
These contaminants must be partially or entirely removed from the industrial streams before such streams can be safely discharged. It will, of course, be understood that this is indeed merely one among very numerous examples of aqueous sources of polluting mercury and organic compounds which are of particular interest for the present invention.
Precipitating agents and flocculants have in the past been extensively used in batch operations for removal of pollutants, especially when colloidal suspensions and emulsions are involved. Precipitating agents are also often used by forming insoluble salts. In the case of semi-soluble organic compounds such as phenols and halogenated hydrocarbons, adsorbents such as activated carbon are among the most common materials used. Large amounts of such materials are often required due to desorption from the carbon because of the relative solubility of these compounds. Phenols and halogenated hydrocarbons are examples of organic materials which behave as organic acids under appropriate conditions. The present invention is in part directed at utilizing these characteristics of acidic organics, such as those mentioned, in order to remove same from the aqueous media in which they are present, and by use of a continuous process as opposed to batch processes.
Although mercury is a naturally occurring element that is present throughout the environment, direct exposure to mercury is harmful to people and wildlife. Power plants and other industrial sources release mercury pollution into the air. In the U.S., coal-fired power plants are the largest source of man-made mercury emissions to the air, accounting for approximately 40% of all mercury emissions. Once in the air, rain and snow deposit mercury into water bodies. Once in the water, mercury bioaccumulates in fish. People and wildlife are exposed to mercury primarily through consuming contaminated fish. Exposure to mercury can lead to toxicological effects in animals and humans, such as neurological and kidney disorders. The most sensitive populations are pregnant woman and their unborn children.
With respect to removal of mercury from aqueous systems such as the mentioned produced water, activated carbon is currently one of the most established mercury control technologies. Among other things different forms of mercury absorb at different rates, in consequence of which activated carbon is not very efficient at removing elemental mercury. Chemical reduction and air stripping has also been used as a low concentration mercury treatment concept for water containing Hg(II). The process consists of dosing the water with low levels of stannous chloride [Sn(II)] to convert the mercury to elemental mercury (Hg0). Hg0 can then be removed from the water by air stripping or sparging. Also known is a process based on photochemical oxidation. This has chiefly been known for use in treating flue gas wherein ultraviolet (UV) light is introduced into the flue gas, to convert elemental mercury to an oxidized form (i.e. mercuric oxide, mercurous sulfate, and mercurous chloride). Once in the oxidized form, mercury can be collected in existing air pollution control devices such as wet SO2 scrubbers, electrostatic precipitators, and baghouses (fabric filters).
None of the foregoing techniques, however, have been fully successful in treating aqueous systems of the type with which the present invention is concerned.